Re: [PATCH 6/7] nouveau/dmem: Evict device private memory during release

[Lyude Paul]

On Mon, 2022-09-26 at 16:03 +1000, Alistair Popple wrote:
> When the module is unloaded or a GPU is unbound from the module it is
> possible for device private pages to be left mapped in currently running
> processes. This leads to a kernel crash when the pages are either freed
> or accessed from the CPU because the GPU and associated data structures
> and callbacks have all been freed.
> 
> Fix this by migrating any mappings back to normal CPU memory prior to
> freeing the GPU memory chunks and associated device private pages.
> 
> Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
> 
> ---
> 
> I assume the AMD driver might have a similar issue. However I can't see
> where device private (or coherent) pages actually get unmapped/freed
> during teardown as I couldn't find any relevant calls to
> devm_memunmap(), memunmap(), devm_release_mem_region() or
> release_mem_region(). So it appears that ZONE_DEVICE pages are not being
> properly freed during module unload, unless I'm missing something?

I've got no idea, will poke Ben to see if they know the answer to this

> ---
>  drivers/gpu/drm/nouveau/nouveau_dmem.c | 48 +++++++++++++++++++++++++++-
>  1 file changed, 48 insertions(+)
> 
> diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c b/drivers/gpu/drm/nouveau/nouveau_dmem.c
> index 66ebbd4..3b247b8 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
> @@ -369,6 +369,52 @@ nouveau_dmem_suspend(struct nouveau_drm *drm)
>  	mutex_unlock(&drm->dmem->mutex);
>  }
>  
> +/*
> + * Evict all pages mapping a chunk.
> + */
> +void
> +nouveau_dmem_evict_chunk(struct nouveau_dmem_chunk *chunk)
> +{
> +	unsigned long i, npages = range_len(&chunk->pagemap.range) >> PAGE_SHIFT;
> +	unsigned long *src_pfns, *dst_pfns;
> +	dma_addr_t *dma_addrs;
> +	struct nouveau_fence *fence;
> +
> +	src_pfns = kcalloc(npages, sizeof(*src_pfns), GFP_KERNEL);
> +	dst_pfns = kcalloc(npages, sizeof(*dst_pfns), GFP_KERNEL);
> +	dma_addrs = kcalloc(npages, sizeof(*dma_addrs), GFP_KERNEL);
> +
> +	migrate_device_range(src_pfns, chunk->pagemap.range.start >> PAGE_SHIFT,
> +			npages);
> +
> +	for (i = 0; i < npages; i++) {
> +		if (src_pfns[i] & MIGRATE_PFN_MIGRATE) {
> +			struct page *dpage;
> +
> +			/*
> +			 * _GFP_NOFAIL because the GPU is going away and there
> +			 * is nothing sensible we can do if we can't copy the
> +			 * data back.
> +			 */

You'll have to excuse me for a moment since this area of nouveau isn't one of
my strongpoints, but are we sure about this? IIRC __GFP_NOFAIL means infinite
retry, in the case of a GPU hotplug event I would assume we would rather just
stop trying to migrate things to the GPU and just drop the data instead of
hanging on infinite retries.

> +			dpage = alloc_page(GFP_HIGHUSER | __GFP_NOFAIL);
> +			dst_pfns[i] = migrate_pfn(page_to_pfn(dpage));
> +			nouveau_dmem_copy_one(chunk->drm,
> +					migrate_pfn_to_page(src_pfns[i]), dpage,
> +					&dma_addrs[i]);
> +		}
> +	}
> +
> +	nouveau_fence_new(chunk->drm->dmem->migrate.chan, false, &fence);
> +	migrate_device_pages(src_pfns, dst_pfns, npages);
> +	nouveau_dmem_fence_done(&fence);
> +	migrate_device_finalize(src_pfns, dst_pfns, npages);
> +	kfree(src_pfns);
> +	kfree(dst_pfns);
> +	for (i = 0; i < npages; i++)
> +		dma_unmap_page(chunk->drm->dev->dev, dma_addrs[i], PAGE_SIZE, DMA_BIDIRECTIONAL);
> +	kfree(dma_addrs);
> +}
> +
>  void
>  nouveau_dmem_fini(struct nouveau_drm *drm)
>  {
> @@ -380,8 +426,10 @@ nouveau_dmem_fini(struct nouveau_drm *drm)
>  	mutex_lock(&drm->dmem->mutex);
>  
>  	list_for_each_entry_safe(chunk, tmp, &drm->dmem->chunks, list) {
> +		nouveau_dmem_evict_chunk(chunk);
>  		nouveau_bo_unpin(chunk->bo);
>  		nouveau_bo_ref(NULL, &chunk->bo);
> +		WARN_ON(chunk->callocated);
>  		list_del(&chunk->list);
>  		memunmap_pages(&chunk->pagemap);
>  		release_mem_region(chunk->pagemap.range.start,

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat